MIMO (multiple input multiple output) system means the system that improvise transmission/reception efficiency of data using multiple transmitting antennas and multiple receiving antennas. The MIMO technology may be categorized into a spatial diversity scheme and a spatial multiplexing scheme. Since the spatial diversity scheme raises transmission reliability or widens a cell radius through diversity gain, it is suitable for data transmission to a user equipment that moves at high speed. The spatial multiplexing scheme transmits different data simultaneously, thereby raising a data transmission rate without increasing a bandwidth of system.
In MIMO system, each transmitting antenna has an independent data channel. A transmitting antenna may mean a virtual antenna or a physical antenna. A receiver receives data transmitted from each transmitting antenna in a manner of estimating a channel for the corresponding transmitting antenna. Channel estimation means a process for reconstructing a received signal by compensating for distortion of a signal caused by fading. In this case, the fading indicates an effect that strength of a signal rapidly fluctuates due to multipath-time delay in a wireless communication system environment. For the channel estimation, a reference signal known to both a transmitter and a receiver is necessary. The reference signal may be simply named RS or a pilot in accordance with an applicable standard.
Downlink reference signal is a pilot signal for coherent demodulation of such a channel as PDSCH (Physical Downlink Shared CHannel), PCFICH (Physical Control Format Indicator CHannel), PHICH (Physical Hybrid Indicator CHannel), PDCCH (Physical Downlink Control CHannel) and the like. The downlink reference signal may be categorized into a common reference signal (CRS) shared by al user equipments in a cell and a dedicated reference signal (DRS) for a specific user equipment only. The common reference signal may be called a cell-specific reference signal. And, the dedicated reference signal may be called a user equipment-specific (UE-specific) reference signal or a demodulation reference signal (DMRS).
In a system having an antenna configuration (e.g., system according to LTE-A standard capable of supporting 8 transmitting antenna) extended more than that of a conventional communication system (e.g., system according to LTE Release 8 or 9 standard) capable of supporting 4 transmitting antennas, a transmission of a reference signal (i.e., CSI-RS) for a receiving side to acquire channel state information (CSI) is requested.